Monoclonal antibodies to human mammary tumor metastases were tested for reactivity to novel and known tumor associated antigens. The monoclonals were used to immunoprecipitate antigens from a radiolabeled breast tumor metastasis extract. Monoclonal antibody B72.3 immunoprecipitated a high molecular weight complex of approximately 220,000d-400,000d. This complex is composed of high molecular weight glycoproteins containing high levels of sialic acid. B6.2 and four other antibodies immunoprecipitated a 90,000d polypeptide. The four other antibodies cross-react in radioimmunoassay for monoclonal B6.2 but differ in their ability to compete with the binding of B6.2. Two antibodies, B1.1 and F5.5, were shown to differentially react with carcinoembryonic antigen. The high molecular weight complex identified by monoclonal B72.3 has been purified from both the breast tumor metastasis extract and the LS174T colon tumor cell line, using molecular sieving and antibody affinity chromatography, with minimal loss of immunoreactivity. Radioimmunoassays have been established for the quantitation of the antigen detected by monoclonal antibody B72.3. These assays are being used to study human serum and other biological fluids.